1. Field of the Invention
The present invention generally relates to a radiation pattern varying technique for antennas, and more particularly to a variable-directivity antenna with a variable radiation pattern, which is made as small as an ordinary omnidirectional antenna and applicable to various types of information technology equipment, such as cellular phones and data processing devices. The present invention also relates to a method for controlling antenna directivity.
2. Description of Related Art
Along with the drastic advancement in radio communications technology, articles and products making use of wireless technologies have become popular, and great expansion of radio channel capacity is now expected. Especially, many studies have been made to increase the transmission capacity of a radio path by carrying out signal multiplexing over multiple dimensions, including time, space, polarized wave, and code.
Spatial multiplexing is realized by an adaptive array antenna constituted by a plurality of omnidirectional antennas and a vector composition circuit for synthesizing the signals. However, applications of such adaptive array antennas are limited because of size constraint on the adaptive arrays, in which each antenna element has a particular size and a certain space is required between antenna elements. For practical purposes, it is desired for an antenna to be as small as possible so as to be applied to mobile communication terminals.
In general, it is preferable to use a directional antenna with a variable radiation pattern (referred to as a “variable-directivity antenna”), rather than using an adaptive array antenna, in order to reduce the antenna size because a directional antenna uses only a set of antenna elements and a feeder circuit to vary the radiation pattern. Accordingly, the variable-directivity antenna is expected to be a candidate for small size antennas that realize spatial multiplexing. However, not many studies have been made so far for reducing the size of a variable-directivity antenna so far, and development of a miniaturized variable-directivity antenna is desired.
Some examples of a variable-directivity antenna are described in publications. For example, JPA 06-350334 disclosed an antenna device that can change the directivity by mechanically adjusting the positional relation between the antenna element and a reflecting element.
FIG. 1A illustrates the antenna device disclosed in JPA 06-350334, in which a reflecting element 511 is set parallel to the antenna element (or a radiator) 510 attached to a conductive member (such as an auto body). The reflecting element 511 is driven around the antenna element 510 by means of the radiation pattern control means 512, which is comprised of a rotating unit 512a and a coupling arm 512b. The antenna element 510 is electrically connected to a power source 515 via a feeder line or a coaxial cable 514.
By changing the rotating angle of the reflecting element 511, the directivity or the radiation pattern of the antenna can be varied. However, the arrangement of reflecting element 511 rotating around the antenna element 510 causes the size of the antenna device to increase.
FIG. 1B illustrates another example of a conventional variable-directivity antenna, as disclosed in JPA 10-154911, which is capable of electrically switching the directivity. The antenna device disclosed in this publication has a center radiation element 612 placed at the center of a round-shaped outer conductor 610 and a plurality of parasitic elements 614 surrounding the center radiation element 612. At the bottom of each parasitic element 614 is provided impedance load 616 for switching the impedance between high and low. The directivity of the antenna is changed by switching the impedance level of the impedance loads 616. The distance between the center radiation element 612 and the parasitic element 614 is about a quarter wavelength (λ/4), and therefore, the antenna size becomes greater than about 1.6λ.
FIG. 1C illustrates still another example of a conventional variable-directivity antenna, which is disclosed in JPA 2001-24431. The variable-directivity antenna disclosed in this publication has an antenna element A0, to which a radio signal is fed, and variable reactance elements A1–A6 surrounding the antenna element A0, to which radio signal are not fed. These antenna elements A0–A6 are arranged on a round-shaped outer conductor 700. The distance “d” between the antenna element A0 and the variable reactance elements is about λ/4, and the size of the entire antenna device becomes about λ.
With the conventional variable-directivity antennas described above, the antenna size inevitably becomes large, as compared with omnidirectional antennas, and accordingly, it is difficult for them to be assembled into compact size information technology equipment, such as cellular phones or portable data processing terminals. This drawback limits applications of variable-directivity antennas.
Especially when the operating frequency is at or below several GHz, the wavelength becomes 10 cm or more, and even a slight change in size affects the handiness of equipment. Due to this drawback, the conventional variable-directivity antennas cannot be applied to mobile communication terminals.